Telnet SUPDUP option
A Telnet option for agreeing to use the SUPDUP display protocol.
Crispin, M.R., "Telnet SUPDUP option", RFC 736, DOI 10.17487/RFC0736, October 1977, <https://www.rfc-editor.org/info/rfc736>.
@misc{rfc736,
series = {Request for Comments},
number = 736,
howpublished = {RFC 736},
publisher = {RFC Editor},
doi = {10.17487/RFC0736},
url = {https://www.rfc-editor.org/info/rfc736},
author = {M.R. Crispin},
title = {{Telnet SUPDUP option}},
pagetotal = 1,
year = 1977,
month = oct,
} RFC text
NWG/RFC# 736 MRC 31-OCT-77 23:28 42213
Telnet SUPDUP Option
Network Working Group Mark Crispin
Request for Comments 736 SU-AI
NIC 42213 31 October 1977
TELNET SUPDUP Option
1. Command name and code.
SUPDUP 21
2. Command meanings.
IAC WILL SUPDUP
The sender of this command REQUESTS permission to, or confirms
that it will, use the SUPDUP display protocol
IAC WON'T SUPDUP
The sender of this command REFUSES to use the SUPDUP protocol.
IAC DO SUPDUP
The sender of this command REQUESTS that the receiver use, or
grants the receiver permission to use, the SUPDUP protocol.
IAC DON'T
The sender of this command DEMANDS that the receiver not use the
SUPDUP protocol.
3. Default.
WON'T SUPDUP
DON'T SUPDUP
i.e., the SUPDUP display protocol is not in use.
NWG/RFC# 736 MRC 31-OCT-77 23:28 42213
Telnet SUPDUP Option
4. Motivation for the option.
Since the publication of RFC 734, I have been requested to design
an option to the TELNET protocol to provide for SUPDUP service.
This option allows a host to provide SUPDUP service on the normal
TELNET socket (27 octal) instead of 137 (octal) which is the normal
SUPDUP ICP socket.
5. Description of the option.
A user TELNET program which wishes to use the SUPDUP display
protocol instead of the NVT terminal service should send an IAC DO
SUPDUP. If the server is willing to use the SUPDUP display
protocol, it should respond with IAC WILL SUPDUP; otherwise it
should refuse with IAC WONT SUPDUP.
For hosts which normally provide SUPDUP terminal services, the
server can send IAC WILL SUPDUP upon ICP which the user may then
accept or refuse.
If the SUPDUP option is in effect, no further TELNET negotiations
are allowed. They are meaningless, since SUPDUP has its own
facilities to perform the functions that are needed. Hence, octal
377 will become an ordinary transmitted character (in this case an
invalid %TD code) instead of an IAC.
Following the mutual acceptance of the SUPDUP option, the SUPDUP
negotiation proceeds as described in RFC 734.
ISO Plain Language
On this page
§1The SUPDUP Option
RFC 736 assigns the Telnet option name SUPDUP the code 21. The option lets a host offer SUPDUP terminal service over the normal Telnet socket instead of the normal SUPDUP ICP socket. It is a negotiation mechanism; the SUPDUP protocol itself is described elsewhere.
The option gives Telnet a named signal for requesting SUPDUP display service. It lets the hosts decide whether to use that service on their connection.
§2How the Hosts Agree
A user Telnet program that wants SUPDUP sends IAC DO SUPDUP. A willing server replies with IAC WILL SUPDUP; otherwise it refuses with IAC WON'T SUPDUP. A server that normally provides SUPDUP may offer the option when the initial connection protocol begins, and the user may accept or refuse.
Once the option is active, the RFC says no further Telnet negotiations are allowed because SUPDUP has its own facilities for needed functions. SUPDUP negotiation then proceeds as described in RFC 734.
The user side asks to use SUPDUP, and the server accepts or refuses. Once both accept, they use SUPDUP's own rules instead of negotiating more Telnet options.
§3The Default State
The default is WON'T SUPDUP and DON'T SUPDUP. In that state, the SUPDUP display protocol is not in use. The option takes effect only after negotiation.
SUPDUP does not start automatically. The connection stays outside SUPDUP unless the hosts agree to turn the option on.